In early May 2008, Cyclone Nargis caused widespread flooding in Burma (Myanmar). Besides trying to assist the storm survivors, the international community also tried to determine the extent of flooding, but making such a determination can be difficult. Clouds often obscure satellite observations, and muddy-brown floodwaters typically blend in with the surrounding landscape, especially fallow cropland. A standard satellite image can provide an approximation of the flood extent, but not an exact outline.

Researchers at the University of Maryland’s Department of Geography developed a method of detecting floodwater by examining multiple sources of information to provide a better—though not perfect—picture of flooded areas. This image combines observations taken on May 5, 2008, by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors on NASA’s Terra and Aqua satellites. It uses the cloud-free observations from each to get the clearest possible picture. Even so, cloud cover obscured both satellites’ views in some areas, so additional flooding could be present. This image also incorporates a percent tree cover dataset called the MODIS Vegetation Continuous Fields. Flooded areas are blue, cities are red, and tree cover appears in shades of green (dense tree cover) to pale yellow (sparse tree cover).

In this image, flooding appears to be more intense in areas with fewer trees. There are different possible explanations for this. Because trees block a satellite’s view of the land surface below, the trees could be hiding floodwaters. On the other hand, in deltas and other low-lying areas that flood often, trees often thrive better on locally higher spots in the terrain, which are less likely to flood. Additional analysis would be needed to determine what connection, if any, exists between the flooded areas, elevation, and tree cover. By examining Terra and Aqua observations, as well as vegetation cover, University of Maryland researchers concluded that Cyclone Nargis flooded about 14,402 square kilometers (5,561 square miles) in the Irrawaddy River Delta in Burma, an area the size of Connecticut.

Image by Robert Simmon based on data from the Department of Geography, University of Maryland. The flood extent product shown here was made possible, in part, by funding from NASA (NASA.NNX08AH05A PI John Townshend) and through the use of NASA MODIS satellite data products. The MODIS Vegetation Continuous Fields data set is available from the Global Land Cover Facility. Caption by Michon Scott and Rebecca Lindsey, based on interpretation by Mark Carroll, Department of Geography, University of Maryland.